Inverter Output Circuit and DC GEC requirements [RE-wrenches]

[Bill Brooks]

Mike,

Mike,

What the code allows and what is good design can be two very different
things. That is why I don't like people getting overly focused on the fact
that the dc is technically a separately derived system according to the
code. That idea will lead you, and many jurisdictional authorities down the
wrong road from a design point of view. It is critical that the dc is
referenced to the exact same grounding source as the ac to prevent the
transistors in the inverter from being fried in a transient (e.g.
lightning).

For a standard building 208Vac separately derived ac system fed from a
480Vac main panel, grounding to building steel is no big deal, because a big
dumb transformer is between the two ac systems. That is simply not the case
with a PV inverter. With PV there are very sensitive power transistors that
work exactly like surge arrestors when the dc circuit ground goes to one
voltage while the ac is at another voltage (they are very expensive surge
arrestors, however). NEC 690.47(C) should not be read as "permitted", but it
really should be read "must". Electrically it is not dangerous to have two
separate grounds--unless you are the inverter--then it is deadly.

If a code inspector requires you to derive a separate ground (690.47(C)(1)),
ALWAYS tie that ground to the ac grounding electrode. The bummer to this
method is that you must use the larger of the two GECs to run between the
two grounding systems (you are creating a grounding electrode system). If
you hold off and tie into the ac grounding electrode (690.47(C)(2)), the
conductor only has to be sized according to the dc side, which is almost
always smaller.

Bill. 

Bill Brooks, PE
Brooks Engineering
873 Kells Circle
Vacaville, CA 95688
office and mobile phone: 707-332-0761
fax:707-451-7739
email: billbrooks7 at yahoo.com
web: www.brooksolar.com

Ecclesiastes 11:7 Light is sweet, and it pleases the eyes to see the sun.


> -----Original Message-----
> From: Mike Bernhard [mailto:spaight12 at sbcglobal.net]
> Sent: Monday, June 18, 2007 4:51 PM
> To: RE-wrenches at topica.com
> Subject: Re: Inverter Output Circuit and DC GEC requirements [RE-wrenches]
> 
> 
> Randy,
> The answer to your first question is no.  Wire, including NM sheathed,
> must
> be protected.
> Question  Don't think of it as a DC GEC.  It is the GEC for the primary
> side
> of the isolation transformer as well as whatever else you would like to
> call
> it  Separately derived systems in  Art 250.  The GEC can be attached to
> ANY
> building grounding electrode including cold water pipe (within 5 ft of
> where
> it comes into the building), building steel, Ufer, ground rod, plate, etc.
> Once again, Art 250, separately derived systems.
> Mike Bernhard
> ----- Original Message -----
> From: "Randy Hatch" <randy at third-sun.com>
> To: <RE-wrenches at topica.com>
> Sent: Monday, June 18, 2007 11:01 AM
> Subject: Inverter Output Circuit and DC GEC requirements [RE-wrenches]
> 
> 
> >
> >
> >            I was hoping that someone could help me to clear up a few
> items
> > that I've been researching.
> >
> >
> >
> > 1)       Is the inverter output circuit conductor (utility interactive
> > inverter) required to be in conduit?  NEC reference?
> >
> > 2)       Is the DC GEC required to be run from the inverter to:
> >
> > a.       The DC GE and then bonded to the AC GE (690.47(1))
> >
> > b.       the AC GE (690.47(2))
> >
> > What should I do for school and commercial installations where providing
> a
> > dedicated DC GEC from the inverter/s to the AC grounding electrode
> system
> > can be considerably more difficult in terms of pathing, distance, etc...
> > Is
> > there a work-around?  NEC reference?
> >
> >
> >
> > Thanks.
> >
> >
> >
> > Randy Hatch
> >= 


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